Since the isolation of camptothecin from the Chinese tree, Camptotheca acuminata, by Wall and co-workers (J. Am. Chem. Soc., 1966, 88, 3888), there had been many approaches to synthesize camptothecin. However, the development of camptothecin as an effective antineoplastic agent was unsuccessful due to its severe toxicity in the first clinical trial in 1970.
Thereafter, Liu et al. reported in 1985 that camptothecin had a specific mode of action to inhibit topoisomerase I. Thus, considerable interest has focused on this compound. In particular, since topoisomerase I inhibitor itself was not used clinically, it is expected that with its actual development, camptothecin may be very efficiently used in combination with other antineoplastic chemotherapeutics having different modes of action.
Recently, various studies for the development of camptothecin derivatives have been proposed in order to reduce the toxicity of camptothecin and to further enhance its antineoplastic activities. Among these related studies, the clinical trial of CPT-11 (Irinotecan) synthesized by Yakurt-Honsha Co. of Japan in 1986 showed that it exhibited excellent antineoplastic activities with less toxicity (Japanese Patent Laid Open Publication No. 64-61482) and followed by other pharmaceutical companies such as Smithkline Beecham (Topotecan) and Glaxo (MDO-camptothecin and 9-aminocamptothecin). Among them, CPT-11 is launched. In particular, it is noticeable that said CPT-11 compound has exhibited excellent antineoplastic activities in the treatment of incurable solid tumors such as the lung cancer.
Since a majority of camptothecin derivatives developed hitherto were semi-synthetic compounds which were obtained from the chemical modification of camptothecin, there were some difficulties in procuring camptothecin and in developing derivatives with a variety of structures because of restrictions of said chemical modification. In addition, these reported total synthesis are not satisfactory in view of both chemical and optical yields.